1. Field of the Invention
The present invention relates to a filter circuit capable of removing common mode noise.
2. Description of the Related Art
FIG. 6 is a circuit diagram showing a conventional low-pass filter circuit 21 called an anti-aliasing filter. The low-pass filter 21 is provided at a previous stage of an ADC (Analog/Digital Converter) to remove high-frequency components (unnecessary signals other than a sampling frequency) from an audio signal inputted to the ADC. However, in the low-pass filter circuit 21, there is a problem that when common mode noise occurs between a ground line connected between a ground GND (AN) connected to an input signal source and a ground GND (AD) connected to the ADC, and an audio signal line between the input signal source and input terminals of the ADC, the common mode noise is superimposed on the audio signal inputted to the ADC. The GND (AN) is an analog ground, and the GND (AD) is a ground of the ADC.
On the other hand, in Japanese Patent Application Laid-Open No. 6-61853, a circuit that removes common mode noise is described. In this literature, there is described a technique of interposing, in a signal output system from a D/A converting unit to an analogy system, differential amplification means A1 with a level of a ground line GNDA used as a reference level to remove an in-phase noise component. Moreover, in Japanese Patent Application Laid-Open No. 9-266417, a ground isolation circuit is described.
Here, if the low-pass filter circuit 21 in FIG. 6 is configured so that the differential amplification means or the isolation circuit described in their Patent Literatures are simply combined to thereby remove the common mode noise, the number of operational amplifiers is disadvantageously increased (i.e., the number of operational amplifiers is increased by that of the operational amplifier as the differential amplification means), resulting in increases in circuit size and cost.